In devices comprising a laser source and coherent heterodyne detection, it may be necessary to determine a frequency change (Doppler effect) produced on part of the path of the laser beam and resulting from a physical phenomenon that is to be compensated for or measured. This is in particular the case of lidars serving to measure the speed and direction of the wind by backscattering of the laser beam from aerosols carried by the wind. This frequency change is generally determined by mixing the signal received with a signal generated by a local oscillator and affected by a frequency shift which is produced by a modulator of the electrooptic or acoustooptic type (AOM).
Devices of this type are also used for measuring the speeds of aircraft in relation to the surrounding medium. The device is known in this application as an “anemometer”.
The measuring distance defines the type of detection of the Doppler shift to be measured and the power of the light source of the anemometer and therefore detection may be either coherent detection or direct or incoherent detection.
In the case of coherent heterodyne detection, the beam coming from the light radiation source (laser) is split in two, one part being shaped spatially and conveyed into the measuring zone. An acoustooptic modulator shifts the frequency of the beam of the reference channel. The backscattered signal is then mixed with the shifted reference so as to generate interference in a detector.
In the anemometer application, a laser beam, generated by a laser source, is focused at a certain distance from the aircraft. Aerosols present in the atmosphere backscatter the incident beam producing a shift of its emission frequency. The Doppler frequency, that is to say the shift between the frequency of the backscattered beams and the incident beam is detected by an interferometer in order to deduce the speed of the aircraft. It is known that the Doppler frequency Fd has the value of:Fd=2v/λ
v being the projection, onto the line of sight of the laser, of the aircraft speed relative to the ambient medium (atmosphere), that is to say the reference with respect to which the speed of movement of the aircraft is to be measured, λ being the wavelength of the emitted beam.
FIG. 1 shows a block diagram of a Doppler shift measuring device of the optical type, of the prior art.
The device of FIG. 1 comprises in particular a laser unit 10, a mixing and detection unit 12 and an optical head 14, these elements 10, 12, 14 corresponding to the main functions of the measuring device.
The laser unit 10 comprises a laser source SL and a polarization-maintaining coupler (PMC) 18 delivering a first optical signal for accessing a signal channel 20 and a second optical signal for accessing a reference channel 22.
The first optical signal in the signal channel 20 is amplified by an optical amplifier (Amp) 26 delivering an optical power signal to be emitted into the reference medium.
The optical power signal leaving the signal channel 20 accesses, via a polarization-splitting coupler (PSC) 32 and a bidirectional optical link 34 of the mixing/detecting unit 12, the optical head 14 radiating a laser beam Fem into the reference medium.
The optical head 14 ensures, on the one hand, focusing of the emitted laser beam Fem in the reference medium and, on the other hand, captures the rays Frd backscattered by the medium in a set direction.
The backscattered rays Frd captured by the optical head, possibly containing a Doppler shift, are conveyed by the bidirectional optical link 34 to the polarization-splitting coupler 32 which delivers a backscattered optical signal Pr, on account of the rotation of the polarization of the backscattered signal relative to the emitted signal, by means of a λ/4 optical plate 35 at a signal return output Sr.
The detection/mixing unit 12 additionally includes a polarization-maintaining coupler (PMC) 40 receiving, at one of its inputs Es, the reference signal Pol leaving the reference channel of the laser unit 10 and, at another input Er, the backscattered signal Pr. The PMC coupler 40 mixes the reference signal and the backscattered signal, possibly containing the Doppler shift, generating interference signals applied to a detector Dt 42.
Signal processing applied to the detector Dt 42 then makes it possible to extract, from the Doppler shift, the measurement of speed of movement v.
In other structures (not shown) detection of the Doppler shift may be of the heterodyne type and, to this end, a frequency shift of one of the two channels is achieved with the aid for example of an acoustooptic modulator (frequency shift by the AOM).